Method of providing and developing hidden entries

ABSTRACT

Paper-based feedback systems containing hidden entries comprising an image forming reactant selected from the group consisting of dithiooxamide, the N,N&#39;&#39;-disubstituted derivatives of dithiooxamide and an iron-complexing compound having a benzene ring and at least two hydroxyl compounds substituted in adjacent positions on said ring. The hidden entries are developed by contacting the entries with an image-forming coreactant contained in a wax matrix in the form of a crayon or sheet.

i United States Patent [191 Fraik Nov. 18, 1975 METHOD OF PROVIDING ANDDEVELOPING HIDDEN ENTRIES [75] Inventor: Robert D. Fraik, LincolnTownship,

Washington County, Minn.

[44] Published under the Trial Voluntary Protest Program on January 28,1975 as document no. B 322,621.

Related us. Application Data [60] Division of Ser. No. 23,036, March 26,1970, Pat. No. 3,725,104, which is a continuation-in-part of Ser. No.822,287, May 6, 1969, abandoned.

52 US. Cl 427/145; 35/9 R; 101/469; 101/470; 427/150; 250/316 51 Int.Cl. B41M 5/12; B41M 5/18 [58] Field of Search 35/9 R; 101/426, 469, 470;

[56] References Cited UNITED STATES PATENTS 2,168,098 8/1939 Groak117/36.2 I 2,755,201

7/1956 Webber et a1. 117/36 2,949,381 8/1960 Steinhardt 117/36 3,076,4062/1963 Florence 101/469 3,170,395 2/1965 Gundlach et a1. 101/4693,262,386 7/1966 Gordon 101/469 3,363,338 1/1968 Skinner et al 35/363,363,556 1/1968 Shely et a1. 101/469 3,386,379 6/1968 Gundlach et a1.101/469 3,451,143 6/1969 Thomas et a1. 101/469 3,476,578 11/19'69Brinckman l17/36.2 I 3,481,760 12/1969 Clark et a1...... 117/36 23,617,324 11/1971 Thomas 117/1.7 3,632,364 1/1972 Thomas et a1. 117/1,73,672,842 ,6/1972 Florin 23/253 TP 3,725,104 4/1973 Fraik 1l7/l.7

Primary Examiner-Michael Sofocleous Attorney, Agent, or FirmAlexander,Sell, Steldt &

. DeLaHunt 57 ABSTRACT Paper-based feedback systems containing hidden 9Claims, N0 Drawings METHOD OF PROVIDING AND DEVELOPING HIDDEN ENTRIESNo. 822,287, filed May 6, 1969, now abondoned.

This invention relates to hidden entry technology.

That is, this invention relates to a method of providing a storablelatent image which can easily be developed at a later time. Theinvention finds utility in education, toys, games, and any other fieldin whicha latent image or pattern of indicia should be invisible to thenaked eye until a desired time (e. g. when the student or playerresponds or chooses a response to a problem or a game situation). I I

Most of the development inthe field of hidden entry printing hascenteredaround educational applications. See, for example, U.S. Pat. No.3,363,336, to B. F. Skinner, issued Jan. 16, 1968. Various training andtesting procedures, particularly the more complex ones, require feedbacksystems. Feedback,has been defined as information transfer from theinstructional material to the student after a response is made by thestudent; see page 1 of a report sponsored by The Southwest RegionalLaboratory for. Educational Research & Development (the SWRL), by JoelStrand berg, Ed. D., entitled Feedback Systems for Use with Paper-BasedInstructional Products, dated Feb. 14, 1969. Paper-based feedbackproducts are, generally speaking, receptor or copy-sheets, cards, or thelike, containing hidden information which can be chemically, orphysically revealed. As compared to mechanical or electrical visualdevices such as teaching machines, paper-based products are economicaland relatively simple to manufacture and use. Some paperbased productsare limited to imparting true/false or yes/no feedback and hereinafterare said to have coded feedback capability, others can feed backdetailed information, thereby stimulating high levels of reasoningleading to a subsequent response, and hereinafter are said to havelatent image feedback capability. In the former systems, only a colorcode or a property of the ink or background of the receptor sheet ishidden; in the latter, an entire printed text is hidden. Needless tosay, latent image feedback capability can be adapted to provide codedfeedback capability, but not vice versa. The prior art systems,regardless of feedback capability, are based upon mimeographing, spiritduplicating, electrostatic photocopying, or high-production printingtechnology. A number of difficult problems are encountered when thesetechniques are adapted to paper-basedfeedback systems.

One of the most difficult problems is to devise a means for providing acopy or receptor sheet with truly invisible and indetectible latentimages or latent coded feedback.

A second, closely related problem is the visibility or legibility of thefeedback. The relationship between the first and second problem is this:hidden images or codes which are truly invisible tend to be poorlylegible upon development, and, conversely, the more legible or visiblefeedback systems are not free of visual cues prior to development. Inother words, solutions to the-two problems tend to be mutuallyexclusive.1

A third. problem concerns the efficiency of-obtaining the feedback. Theideal system would be virtually in- I stantaneous. Unfortunately, somechemical systems are slow, involving reaction times of a second or more.When an image is physically hidden by an overlay of ink, varnish, powderor the like, the overlay must be laboriously scraped off or erased, aneven slower and certainly more cumbersome process of obtaining feedbackthan most chemical systems.

A fourthproblem is the adaptabllity of the system to short'runs andsmall-scale production. Ideally, educa tional tests should be revisedfrequently and should be economically reproduced in small quantities.Most printing (as opposed to duplicating) methods, even offset andletterpress techniques, require large runs to be economical. I

A fifth problem relates to the extent of adaptation required to converta printing, mimeographing, duplicating, or photocopying technique topaper-based feedback systems. With some systems, equipment which mayalready be available to the educational institution cannot be used assuch, but must be specially modified in order to produce paper sheetswith feedback capability. Ideally, an inexpensive, short-run productionsystem should be capable of being used with unmodified, existingequipment. Likewise, it is obviously desirable that the feedback systembe operative using ordinary, untreated paper, and the means fordeveloping, revealing, or activating the feedback should be simple andeconomical.

Two proposed chemical systems have latent image feedback capability.Both involve revealing the latent image through the use of a chemicalpen containing a co-reactant for the active chemical component of theimage. In both systems, it is very difficult to avoid the twin dilemmaof visible cues prior to image development or illegible images afterdevelopment. One system utilizes letterpress printing and the other is aspirit duplicating process. The spirit duplicating method has theadvantage of being adaptable to short-run production, but it appears tohave a tendency to involve long reaction times and to produce ratherweakly visible images. The chemistry and procedural steps of spiritduplicating can be found in U.S. Pat. No. 3,076,406, issued Feb. 5, 1963to Robert T. Florence.

Accordingly, this invention contemplates a paperbased feedback systemutilizing hidden entry printing wherein a truly invisible latent image(printed on a receptor sheet by equipment now available and capable ofeconomical short-run production) is rapidly and efficiently made visiblewith a chemical reactant.

This invention also contemplates a method of providing the feedbacksystem and an easy to use, economically made writing instrument fordispensing the chemical reactant which makes the image visible.

Briefly, this invention involves adapting the technology ofthermographic reproduction to paper-based feedback systems. Typicalthermographic processes are described in U.S. Pat. No. 3,094,417, issuedJune 18, 1963 to Workman and U.S. Pat. No. 3,280,735, issued Oct. 25,1966 to Clark etal. Thev Clark et al process,

ing to the master image is transferred to the receptor sheet.

In the latent image printing process of this invention, the vaporizable,heat transferable chemical alone does not produce a visible image. Thevapor condenses on the surface of the receptor, which can consist of anuntreated sheet of ordinary bond paper or punchcard stock, and therebyforms a stored, latent image. At a later time, as much as several weekslater, a chemical co-reactant is applied to the image area of thereceptor sheet. The two chemicals react almost instantaneously to form astable, colored complex and the image thereby becomes clearly visibleand legible.

It is quite surprising that the thermoplastic reproduction technologyof, for example, the Clark et al U.S. Pat. No. 3,280,735 can be adaptedto the field of paper-based feedback systems and hidden entry printing.Thermographic systems generally contemplate the use of a coated receptorsheet coated with a normally stably solid imaging reactant (i.e., dyeprecursor) which is easily vaporized or sublimed, or some other imagingmeans, e.g. a heat transferable, visible dye, for producing animmediately visible, rather than a latent, image. It would be expectedthat the heat-transferable chemical condensed on the receptor sheetwould have poor storage characteristics, i.e., poor shelf life. It wouldfurther be expected that, as compared to spirit systems, a thermographicsystem would produce an inferior, relatively low-contrast image upondevelopment with a co-reactant, regardless of the chemistry of thesystem. For example, in the spirit duplicating system referred topreviously, the latent image-forming chemical is leached onto thereceptor with a solvent, and it would be expected that far more of thischemical would be transferred to the receptor or copy sheet than is thecase with heat-transfer systems. Surprisingly, however, the latentimages produced by this invention are storable for long periods of timeand produce relatively high-contrast visible images upon development.

The preferred method of this invention comprises the following steps:

1. Providing a master or original for reproduction of the latent image:This step essentially involves printing, typing, or otherwise inscribinga master or original sheet with the intelligence which will be containedin the feedback. The image areas of the master will have a greatertendency to absorb heat or infrared energy than the uninscribed orbackground areas. Generally speaking, white, red and otherinfrared-reflecting colors provide suitable backgrounds, and black orother infrared absorbing colors provide suitable image areas. The masteror original can be inscribed with typing, handwriting, photocopying,printing, or any other method which produces infrared absorbing imagesand a high contrast with the poorly-absorptive background. Notes orinstructions not to be included in the latent image can be inscribed onthe surface of the master in non-absorptive colors such as red or white.

2. juxtaposing original or master, vapor-supply medium, and receptorsheet: The imaged original or master is preferably placed on top of areceptor sheet which preferably is already inscribed with visibleintelligence. That is, the receptor sheet will normally have beenprovided, by any suitable method, with visible instructional or otherprinted information, questions, pictorial representations and the likewhich will be arranged to be in register with the latent images to beformed. Alternatively, this visible image material can be provided byany means which does not develop the latent image, including thecomposite master system described subsequently. The background areas ofthe receptor sheet can be any color which provides high contrast withthis visible material and the developed latent image, e.g. white oryellow. The most economical type of receptor sheet would consistessentially of ordinary bond paper, punchcard stock, or the like free ofspecial coatings or other expensive treatments. However, the inventionis not limited to untreated receptor sheets. Coatings or othertreatments which do not render visible the latent image are consistentwith the objectives of this invention. See, for example, U.S. Pat. No.3,481,759 to D. A. Ostlie, issued Dec. 2, 1969, which describes the useof zinc salts in receptor sheet coatings. Zinc salts react withdithiooxamide or its derivatives to produce white or colorless complexeswhich are substantially invisible on a white background. It is alsoknown that gallic acid and gallic acid esters, pyrogallol, catechol, andsimilar polyhydroxy benzene compounds form weak, colorless complexeswith nickel salts. Interposed between the original or master and thereceptor sheet is a layer of normally stably solid vaporizable reactantmaterial, such as dithiooxamide, its derivatives, esters of gallic acid,and other materials described hereinafter. The layer can be in the formof a separate vapor supply sheet or a coating on the reverse side of theoriginal. That is, the imaged original may comprise a sheet which isimaged with radiation-absorptive material on the first major surface andhas a layer of the vaporizable reactant material on the second majorsurface.

The coating on the second major surface of the original sheet need notbe coextensive in area with this surface but can consist of localizeddeposits. For example, the layer of normally stably solid vaporizablereactant material can be a mirror image of the image inscribed, typed,printed, etc., on the first surface of the master or original. Thismirror image layer is provided as follows: (a) An original or master isplaced against the transfer coating of a transfer sheet, the transfercoating containing a particulate material mixed with the vaporizablereactant material. (b) The first (exposed) surface of the master istyped, written, printed, or otherwise inscribed upon, using sufficientpressure to cause part of the aforementioned transfer coating to bedeposited on the second (reverse) surface of the master in the form of amirror image of the pattern of pressure applied to the first surface.See U.S. Pat. application Ser. No. 781,195, filed Dec. 4, 1968. If themaster or original is an ordinary bond paper, the only vaporizablereactant material on the second surface is this mirror image, andheat-transfer of the mirror image to form a direct latent image on thereceptor sheet can be effected with virtually any heat source, includinginfrared radiation, a heated roll, or a hot plate. If the master alreadyhas a coating or layer of dithiooxamide (or a derivative thereof) on thesecond surface, selective use of a transfer sheet having a transfercoating containing an ortho-dihydroxy or trihydroxy-benzene or -benz icacid or ester (e.g. catechol, pyrogallol, gallic acid, an alkyl gallate,etc.) will provide a composite master capable of vapor transferring twoseparate texts: a dithiooxamide text and, for example, an alkyl gallatetext. A nickel-coated receptor sheet will image the dithiooxamide textimmediately, but the gallate text will remain latent or hidden.Subsequent treatment of the gallate latent images with an iron salt willprovide feedback.

The juxtaposition described above can be achieved by mounting thereverse side-coated master on a rotatable drum of an infraredduplicating machine and using receptor sheets as the feed. n

3. Irradiation: After the imaged original or master, the layer ofvaporizable reactive material, ancl the receptor sheet have beenproperly juxtaposed, the imaged surface of the original or master ispreferably exposed to intense infrared radiation. That is, the master ispreferably between the radiation source and the receptor, thevapor-supply medium being interposed between master and receptor asdescribed in step (2). The imaged portion of the original will naturallytend to absorb more radiation and become relatively hot comparedto, thepoorly-absorptive background (including notes and instructions writtenin non-absorptive ink) or unimaged areas. Vapor will be caused toemanate from the layer of vaporizable reactant material in localizedareas corresponding to the image on the master. The vapor will migrateor transfer to the receptor sheet.

4. Condensation: Since the receptor sheet, regardless of whether it ispunchcard stock or-bond paper, contains no substances which are visiblyreactive with the vapor from the vaporizable layer, the vapor willmerely condense upon the fibers of the receptor'sheet. Alternatively,the vapor will react with a chemical in the sheet which will produce acolorless compound or a colored compound which does not contrast withthe background of the sheet. The result will be a latent imagecomprising stable, solid, invisible deposits of the condensate of (orthe invisible product derived from) vaporizable reactant in localizedareas corresponding to the image areas of the original sheet.

5. Duplicating: Steps (3) and (4) can be repeated with additionalreceptor sheets many times. 'A single master or original sheet iscapable of producing several dozen copies. Due to the ease and low costof imaging a master, several hundred copies can be preparedeconomically. Steps (3) and (4) can be rapidly carried out on existinginfrared copying or duplicating machines. Such machines are constructedand arranged to provide a line source of light including a tubular lamphaving a linear filament and mounted within a focused reflective housingfor progressive exposure of the printed surface of the original, all asdescribed in Miller US. Pat. No. 2,740,895. Another suitable form ofapparatus is described in ,Kuhrmeyer et al. US. Pat. No. 2,891,165.

6. Developing the latent image: The latent image comprising theinvisible deposits referred to previously can be rendered visible bytreating the surface of the receptor sheet with an image-forming (i.e.,a colored chemical complex-forming) co-reactant dissolved'in a solventalso capable of dissolving the vaporizable reactantmaterial. Suitablesolvents include both polar and non-polar organic liquids such as thehydrocarbons, particularly cyclohexane; the alcohols, particularlyethanol; and aldehydes and ketones, such as acetone and methylethylketone. Mixtures of these solvents can be used to control volatilitylThe co-reactant can be dispensed from a writing instrument such as afelt-tipped pen, wherein theink reservoir contains the co-reactant-/solvent system; a wax crayon, wherein the wax itself serves as areservoir, matrix, or solid solution medium for this system; or asheetlike dispensing means such as a saturated blotter or a transfersheet coated with awax system similar to that of the wax crayondescribed previously. For example,'U.S. Pat.- No. 2,168,098 toJ. Groak,issued Aug. 1, 1939 discloses a transfer sheet coated with a metal salt(e.g. an iron salt) admixed with 6 a wax (e.g. an ester wax), apolymeric binder (e.g. starch), and a material to facilitate the imagingreaction between the metal salt and a dye precursor such as gallicacid.The substrate for the coating is preferably flexible (e.g. paper) butcan be more rigid than paper, e.g. a film of an organic syntheticpolymer. Both the aforementioned transfer sheets and the wax crayons areparticularly suitable for shipment with correspondence course materials.The wax crayons can comprise, for example, an organic or inorganic metalsalt, a waxy polymer containing oxyalkylene units (e.g. polyoxyethyleneglycols and esters-or ethers thereof), :and a reaction acceleratorand/or a hardness controlling agent such as tributyl phosphate and/or asecondary amine substituted with two hydroxy-loweralkyl groups (e.g.diethanol amine).

Other means can be used for dispensing the co-reactant and/or solventsand reaction accelerators, e.g., brush applicators,spray applicators,and the like. The development step is accelerated byorganic trialkyl andtriaryl phosphates such as tributyl phosphate and triphenyl phosphate.Tributyl phosphate can also be used in place of the above-mentionedsolvents.

Method steps l (5) described above are preferred because they providegood transfer of vaporizable reactive material to the receptor sheet. Inshort, various heat-transfer processes can be adapted for use with thepresent invention, including those which produce a mirror image andrequire the use of an intermediate transfer sheet and those in which thereverse side of the original or master contains a mirror image ofvaporizable reactant material.

Excellent latent image feedback capability can be provided when thevaporizable reactant material is di-. thiooxamide (hereinafter referredto as DTO) or deriv-, atives of DTO. The preferred co-reactants forthese vaporizable materials are the organic carboxylic acid salts orinorganic salts of metals such as nickel, copper, co,- balt, andcadmium. The preferred DTO derivatives are the N,N.-disubstitutedcompounds, e.g. N,N'-dibenzyl' dithiooxamide (hereinafter referred to as.DBDTO);

N ,N '-dimethyl dithiooxamide; N ,N -2-hydroxyethyl dithiooxamide; andother N ,N '-di-organic radical-substituted DTO compounds. Since thevapor pressure of DTO is higher at room temperature than, that of any.of the N,N-disubstituted DTO compounds, the DTOderivatives, particularlyDBDTO, provide the most lasting, i.e., the most storable, latent images-The organic carboxylic acid anions of the preferred metal saltco-reactants are preferably acetate, rosinate,

oleate, 2-alkyl-hexanoate, or naphthenate. Other long chain acid anionsbesides the rosinate and oleate are operative, e.g. the fatty acidanions such as stearate anion.

A receptor sheet properly provided with a latent image of one of thepreferred vaporizable reactant ma terials can be developed with apreferred co-reactant to produce a dark, legible image weeks or evenmonths after this receptor sheet has been prepared. Yet, prior todevelopment, the latent image cannot be detected even under amicroscope. DBDTO forms a latent image which can be developed to avisible image with a nickel salt from 3 to 10 months after thecondensation step, provided the receptor sheet is kept in a closed fileor drawer. Even when a DBTO-imaged receptor sheet is mishandled, i.e.,left exposed to freely-circulating-air rather than kept within a closedspace, weak but visible images can be developed even after 6 weeks ormore of 7 such mishandling.

Other chemical systems are adaptablefor use with this invention. Forexample, dimethylglyoxime provides a latent image which is somewhat lessstorable but otherwise excellent. The known ortho-dihydroxyortrihydroxyaromatic imaging compounds are also operative, e.g. gallicacid and its esters, pyrogallol, catechol, and derivatives of these,e.g. higher acyl-substituted pyrogallols. These ortho-diortrihydroxyaromatic compounds do not provide colored complexes whenreacted with nickel salts. However, they are known to coreact with ironsalts to produce strongly colored complexes. Both inorganic iron saltssuch as ferric chloride or ferric nitrate and organic salts such asferric benzoate or ferric octoate are suitable coreactants. Diphenylcarbazone, S-diphenyl carbazide, a,a-dipyridyl, protocatechuic acid,S-hydroxyquinoline, oxalic acid, I- hydroxy-4-methyoxy naphthalene,amonium thiocyanate, National Aniline Color Precursors Nos. 1, 2, and 4,thiourea, l-naphthaldehyde benzoylhydrazone, the metal behenates,octadecyl amine vanadate, methyl orange etc. can be used but tend to beless suitable to this invention.

Thus, the preferred embodiment of this invention provides a printed formsuitable for use in paper-based feedback systems wherein part of theprinting is visible (e.g. test instructions and the like) and otherparts are hidden entries comprising latent images.

The following non-limiting example illustrates the principles andpractice of this invention.

EXAMPLE Twenty-five sheets of a commercially available bond paper weremimeographed with a text setting forth a medical school examinationproblem. Opposite a list of visible, mimeographed entries on the bondsheets, spaces were provided for hidden entries. The top surface of amaster was. imaged with infrared-absorbing ink. The images consisted oftextual feedback arranged to provide the hidden entries in the spaces onthe mimeographed sheets, the bottom surface of the master being a DBDTOcoating. The mimegraphed side of a bond sheet was placed against thebottom surface of the master, and the juxtaposed sheets were fed to aninfrared thermographic copying machine (3M Model 47 Therrno-Fax copier,also known as the Secretary). The juxtaposing and copying steps wererepeated with the master and twenty-five bond sheets to provide 25copies of the examination problem, with hidden entries in register withthe text of the problem and in the appropriate spaces. A representativesampling of the latent-imaged bond sheets was examined under amicroscope, but no latent images could be detected.

The reservoir of a felt tipped pen was filled with nickel rosinatedissolved in tributyl phosphate and cyclohexane. A few minutes after thefirst bond sheets had been run off on the thermographic copying machine,the spaces opposite the visible entries on this sheet were rubbed withthe tip of the felt pen. Dark purple images corresponding to the masterimages were obtained almost instantaneously.

About half of the remaining, undeveloped bond sheets were placed in atile folder which was kept in a closed file cabinet for 8 weeks. Atvarious times during the 8-week period, sheets were removed and rubbedwith the felt pen as before. In every case, dark, easily legible, purpleimages were obtained. A similar image was obtained when one of thesheets was rubbed with a 8 wax crayon containing nickel rosinate andtriphenyl phosphate and another sheet was rubbed with a falt pencontaining a nickel 2-ethylhexanoat e solution.

The remaining hidden entry-containing bond sheets were allowed to lay ona table top in a well-lighted and -ventilated room. After 6 to 8 weeks,weak but legible blue or purple images could still be obtained witheither of the felt pens described previously.

The person skilled in the art will appreciate that many modifications ofthis invention are possible. For example, due to the high contrast ratioof the developed latent image to the background of the receptor sheet,instructional materials prepared according to this invention could beadapted for use with machine grading systems employing photoelectricsensing. Such machine grading systems would merely be required to detectthe presence or absence of a developed image.

What is claimed is:

l. A method of developing a latent image to provide feedback in apaper-based feedback system, said latent image comprising animage-forming reactant material selected from the group consisting ofdithiooxamide, the N,N-disubstituted derivatives of dithiooxamide, andan iron-complexing compound capable of forming a colored complex whenreacted with the ions of an iron salt, said iron-complexing compoundhaving a benzene ring and at least two hydroxyl groups substituted inadjacent positions on said ring, said method comprising contacting saidlatent image with a solid wax matrix means containing an image-formingcoreactant, to transfer a portion of said image-forming coreactant fromsaid wax matrix means to said latent image and to react and render saidlatent image visible.

2. The method according to claim 1 wherein said image forming coreactantcomprises a metal salt and said wax matrix contains an image-formingreaction accelerator.

3. The method according to claim 2 wherein said reaction accelerator isselected from the group consisting of organic trialkyl phosphates andorganic triaryl phosphates.

4. A method of developing a latent image to provide feedback in apaper-based feedback system, said latent image comprising animage-forming reactant material selected from the group consisting ofdithiooxamide, the N,N'-disubstituted derivatives of dithiooxamide, andan iron-complexing compound capable of forming a colored complex whenreacted with the ions of an iron salt, said iron-complexing compoundhaving a benzene ring and at least two hydroxyl groups substituted inadjacent positions on said ring, said method comprising contacting saidlatent image with a sheet-like means for dispensing an image-formingcoreactant to transfer said image-forming coreactant from saidsheet-like means to said latent image and to react and render saidlatent image visible. V

5. A method of providing and developing hidden entries comprising thesteps'of:

1. providing intelligence in the form of preferentiallyradiation-absorptive image areas upon the surface of a significantlyless radiation-absorptive paperlike original sheet;

2. juxtaposing said original sheet, a layer containing a normally stablysolid vaporizable reactant material, and a receptor sheet, said receptorsheet being essentially free of materials capable of reacting with saidvaporizable reactant material to produce contrasting colored complexes,said normally stably nal sheet to a heat source to induce in the saidimage areas a localized increase in temperature and a correspondinglocalized vapor transfer of said vaporizable reactant material from saidlayer to said receptor sheet;

4. permitting the vapor transferred, vaporizable reactant material tocondense on said receptor sheet, thereby providing a storable latentimage in localized areas of said receptor sheet corresponding to saidimage areas of said original sheet; and

5. contacting said storable latent image with a solid wax matrix meanscontaining an image-forming-co- 10 reactant whereby a portion of saidimage forming coreactant is transferred from said wax matrix means tosaid storable latent image and said image is rendered visible.

6. The method according to claim 5 wherein said layer of normally stablysolid vaporizable reactant material comprises a coating on one majorsurface of the said original sheet; said juxtaposing, exposing, andpermitting steps are repeated with additional receptor sheets to formadditional copies of said latent-imaged receptor sheet.

7. A method according to claim 6 wherein the said coreactant is a metalsalt, the metal of said metal salt being selected from the groupconsisting of nickel, copper, cobalt, and cadmium.

- 8. A method according to claim 5 wherein said original sheet containsa layer of said iron-complexing compound, said layer comprising a mirrorimage of said latent image. 1

9. A method according to claim 5 wherein said heat source is a source ofinfrared radiation.

1. PROVIDING INTELLIGENCE IN THE FORM OF PREFERENTIALLYRADIATION-ABSORPTIVE IMAGE AREAS UPON THE SURFACE OF A SIGNIFICANTLYLESS RADIATION-ABSORPTIVE PAPER-LIKE ORGINAL SHEET, 2 JUXTAPOSING SAIDORIGINAL SHEET, A LAYER CONTAINING A NORMALLY BESTABLY SOLID VAPORIZABLEREACTANT MATERIAL, AND A RECEPTOR SHEET, SAID RECEPTOR SHEET BEINGESSENTIALLY FREE OF MATERIALS CAPABLE OF REACTING WITH SAID VAPORIZABLEREACTANT MATERIAL TO PRODUCE CONTRASTING COLORED COMPLEXES, SAIDNORMALLY STABLY SOLID VAPORIZABLE REACTANT MATERIAL BEING SELECTED FROMTHE GROUP CONSISTING OF: DITHIOOXAMIDE, THE N,N''-DISUBSTITUTEDDERIVATIVES OF DITHIOOXAMIDE, AND A IRON-COMPLEXING COMPOUND CAPABLE OFFORMING A COLORED COMPLEX WHEREIN REACTED WITH THE IONS OF AN IRON SALT,SAID IRON-COMPLEX COMPOUND HAVING A BENZENE RING AND AT LEAST TOHYDROXYL GROUPS UBSTITUTED IN ADJACENT POSITIONS ON SAID RING,
 1. Amethod of developing a latent image to proVide feedback in a paper-basedfeedback system, said latent image comprising an image-forming reactantmaterial selected from the group consisting of dithiooxamide, theN,N''-disubstituted derivatives of dithiooxamide, and an iron-complexingcompound capable of forming a colored complex when reacted with the ionsof an iron salt, said iron-complexing compound having a benzene ring andat least two hydroxyl groups substituted in adjacent positions on saidring, said method comprising contacting said latent image with a solidwax matrix means containing an image-forming coreactant, to transfer aportion of said image-forming coreactant from said wax matrix means tosaid latent image and to react and render said latent image visible. 1.providing intelligence in the form of preferentiallyradiation-absorptive image areas upon the surface of a significantlyless radiation-absorptive paper-like original sheet;
 2. juxtaposing saidoriginal sheet, a layer containing a normally stably solid vaporizablereactant material, and a receptor sheet, said receptor sheet beingessentially free of materials capable of reacting with said vaporizablereactant material to produce contrasting colored complexes, saidnormally stably solid vaporizable reactant material being selected fromthe group consisting of: dithiooxamide, the N,N''-disubstitutedderivatives of dithiooxamide, and an iron-complexing compound capable offorming a colored complex when reacted with the ions of an iron salt,said iron-complex compound having a benzene ring and at least twohydroxyl groups substituted in adjacent positions on said ring;
 2. Themethod according to claim 1 wherein said image forming coreactantcomprises a metal salt and said wax matrix contains an image-formingreaction accelerator.
 3. The method according to claim 2 wherein saidreaction accelerator is selected from the group consisting of organictrialkyl phosphates and organic triaryl phosphates.
 3. EXPOSING THE SAIDSURFACE OF SAID JUXTAPOSED ORGINAL SHEET TO A HEAT SOURCE TO INDUCE INTHE SAID IMAGE AREAS A LOCALIZED INCREASE IN TEMPERATURE AND ACORRESPONDING LOCALIZED VAPOR TRANSFER OF SAID VAPORIZABLE REACTANTMATERIAL FROM SAID LAYER TO SAID RECEPTOR SHEET,
 3. exposing the saidsurface of said juxtaposed original sheet to a heat source to induce inthe said image areas a localized increase in temperature and acorresponding localized vapor transfer of said vaporizable reactantmaterial from said layer to said receptor sheet;
 4. PERMITTING THE VAPORTRANSFERRED, VAPORIZABLE REACTANT MATERIAL TO CONDENSE ON THE RECEPTORSHEET, THEREBY PROVIDING A STORABLE LATENT IMAGE IN LOCALIZED AREAS OFSAID RECEPTOR SHEET CORRESPONDING TO SAID IMAGE AREAS OF SAID ORIGINALSHEET, AND
 4. A METHOD OF DEVELOPING A LATENT IMAGE TO PROVIDE FEEDBACKIN A PAPER-BASED FEEDBACK SYSTE, SAID LATENT IMAGE COMPRISING ANIMAGE-FORMING REACTANT MATERIAL SELECTED FROM THE GROUP CONSISTING OFDITHIOOXAMIDE, THE N,N''-DISULBSTITUTED DERIVATIVES OF DITHIOOXAMIDE,AND AN IRON-COMPLEXING COMPOUND CAPABLE OF FORMING A COLORED COMPLEDWHEN REACTED ITH THE IONS OF AN IRON SALT, SAID IRON-COMPLEXING COMPOUNDHAVING A BENZENE RING AND AT LEAST TO HYDROXYL GROUP SUBSTITUTED INADJACENT POSITIONS ON SAID RING, SAID METHOD COMPRISING CONTACTING SAIDLATENT IMAGE WITH A SHEET-LIKE MEANS TO SAID DISPENSING AN IMAGE-FORMINGCOREACTANT TO TRANSFER SAID IMAGE-FORMING COREACTANT FROM SAIDSHEET-LIKE MEANS TO SAID LATENT IMAGE AND TO REACT AND RENDER SAIDLATENT IMAGE VISIBLE.
 5. A METHOD OF PROVIDING AND DEVELOPING HIDDENENTRIES COMPRISING THE STEPS OF:
 5. CONTACTING SAID STORABLE LATENTIMAGE WITH A SOLID WAX MATRIX MEANS CONTAINING AN IMAGE-FORMINGCOREACTANT WHEREBY A PORTION OF SAID IMAGE FORMING COREACTANT ISTRANSFERRED FROM SAID WAX MATRIX MEANS TO SAID STORABLE LATENT IMAGE ANDSAID IMAGE IS RENDERED VISIBLE.
 6. The method according to claim 5wherein said layer of normally stably solid vaporizable reactantmaterial comprises a coating on one major surface of the said originalsheet; said juxtaposing, exposing, and permitting steps are repeatedwith additional receptor sheets to form additional copies of saidlatent-imaged receptor sheet.
 7. A method according to claim 6 whEreinthe said coreactant is a metal salt, the metal of said metal salt beingselected from the group consisting of nickel, copper, cobalt, andcadmium.
 8. A method according to claim 5 wherein said original sheetcontains a layer of said iron-complexing compound, said layer comprisinga mirror image of said latent image.
 9. A method according to claim 5wherein said heat source is a source of infrared radiation.